This project, which has been continuously funded for 30 years, addresses adipose metabolism that controls whole body glucose and lipid homeostasis in health and disease. Co-morbidities associated with obesity, including insulin resistance and type 2 diabetes, can result from insufficient deposition and storage of triglyceride (TG) within adipose tissue, leading to deleterious effects on liver, skeletal muscle, cardiovascular organs and insulin secreting beta cells. Thus a high capacity of adipose tissue for fatty acid esterification and sequestration of TG is vital during caloric surplus. However, thi function of adipocytes involves complex and incompletely understood interactions with macrophages and other cells of the immune system that infiltrate adipose tissue in obesity. Three key mechanistic questions are: What are the major signaling pathways that regulate fatty acid esterification and TG sequestration in adipocytes? How do macrophages modulate these pathways of adipocyte lipid storage? What controls macrophage infiltration into adipose tissue? This project previously employed siRNA screens to discover the Ste20 ortholog Map4k4, a protein kinase we discovered is integral to all three questions. Based on our recent exciting data, we now hypothesize that 1. Map4k4 in adipocytes, activated by macrophage cytokines, markedly suppresses adipocyte fatty acid esterification and TG sequestration, 2. Knockout of Map4k4 in adipocytes in mice will enhance TG storage in adipose tissue and increase glucose tolerance and decrease fatty liver, and 3. Map4k4 in adipose endothelial cells mediates upregulation of adhesion molecules that promote macrophage extravasation. We propose to identify and define the Map4k4 signaling pathways for lipogenic suppression in adipocytes in vitro (Aim 1) and in vivo (Aim 2) and for activation of endothelial cell permeability (Aim 3). We will define the specific roles of PPARg, SREBP1c and ChREBP as targets of Map4k4 regulation in adipocytes, and utilize two floxed mouse models we already generated wherein Map4k4 is selectively silenced (shRNA) or ablated in adipocytes or endothelial cells, respectively. We will test whether increased lipogenesis in adipose specific Map4k4 KO mice results in increased glucose tolerance, and whether adhesion molecule expression and macrophage extravasation is inhibited in endothelial specific KO mice. These experiments will provide important insights into adipose lipid handling in metabolic regulation and Map4k4 signaling in cross talk among cell types within adipose tissue.